X-51A Screams to Hypersonic Success

A nine-year development effort has finally paid off to create air-breathing planes that can hit Mach 5 and above. PM gets an update from program officials about what this means to the future of flight.

An unmanned experimental aircraft has finally succeeded in doing what no other air-breathing airplane has done before: fly faster than Mach 5, five times the speed of sound, and remain stable until the fuel tanks ran dry.

The X-51A project is managed by the Air Force Research Laboratory, or AFRL, with funding from the Pentagon's mad science division, the Defense Advanced Research Projects Agency, or DARPA. Charlie Brink, the program manager for the project at the AFRL, said in a conference call with reporters on Thursday that while the program had endured three previous failures, the fourth and final flight of the X-51A design, which took place on May 1, was a resounding success. "Our goal," Brink said, "was to demonstrate that you could propel an air vehicle and use scramjet power using hydrocarbon fuel."

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More than that, this flight marked the first time that an air-breathing hypersonic vehicle remained in flight for as long as it had fuel to power it. Previous experimental planes, such as NASA's unmanned X-43, ran for just a handful of seconds at a time before losing thrust. The X-51A ran about 210 seconds, Brink said.

A scramjet works by scooping enough oxygen from the air while in flight to keep up a furious rate of fuel consumption. Achieving this feat—which is often compared to keeping a match burning in a hurricane because of the supersonic wind flowing through the engine—requires that engineers eliminate the moving parts of an ordinary jet engine. For example, jet engines use fans to compress incoming air to feed a high rate of fuel burn. But no known materials can stand up to the speed at which fans would have to turn to push an aircraft to hypersonic speed without flying apart. The fastest jet ever, the SR-71 Blackbird spy plane, could reach Mach 3. Fighter jets can fly around Mach 2.

To get up to supersonic speed, the X-51A dropped from the wing of a B-52 flying out of Edwards Air Force Base in California. Once free, it fired a booster rocket attached to its tail. The booster rocket then dropped from the X-51A and left the scramjet on its own to ignite.

Major Andrew Murphy, the B-52 pilot, told reporters he and his flight crew could feel the vibrations from the rocket booster even though the X-51A was hundreds of feet below them by the time the booster fired. "It is just really impressive to see that thing cook off and go," Murphy said. "When the booster separated and the contrail kept going, we got word from the control room that the scramjet motor was operating on JP-7 fuel. There were a lot of cheers in the B-52."

Each of the four X-51As built for this $300 million program was a one-shot deal. The three other flights, in 2010, 2011, and 2012, all ended in failures of one kind or another. The 2010 flight was the only other one in which the scramjet engine actually ran. In that flight, though, the engine shut down prematurely after 140 seconds, which at the time was the longest any scramjet had flown.

The X-51A design is also the only scramjet to run on hydrocarbon fuel. NASA's X-43 used hydrogen, as have scramjets built by the University of Queensland in Australia. The X-51A ran on JP-7, the aviation fuel used by the SR-71 Blackbird, the idea being to make scramjet technology work with existing aviation infrastructure. Hydrogen fuel, though it packs more energy into the same weight, must be kept cold enough to remain liquid and requires special cryogenic fuel-handling equipment.

The first planned use of scramjet technology is for hypersonic cruise missiles. Unlike rockets, scramjets don't have to carry their own oxygen supply, meaning they can carry more payload on a smaller vehicle. Brink foresees future scramjet-powered missiles the size of the X-51A hitting targets 500 to 600 nautical miles from the launch point within 10 to 12 minutes. The advantage would be a faster strike time by a missile that air defense systems would have a hard time shooting down.

At the earliest, Brink said, the technology might be mature enough for production in 2020. "There's an awful lot of work to be done," he said. Among the challenges still to solve are developing sensors that can withstand the high temperatures caused by the friction from a vehicle moving through the air at hypersonic speed. Although the X-51A project is at an end, it seems certain that other hypersonic projects will follow. Brink wouldn't comment on them, however, as they are still being put together.

Joseph Vogel, the program manager for the X-51A at Boeing, which built the vehicle's airframe, said civilian applications will likely follow the military ones. "If you look historically, after things are used for military applications, there's a couple of years and it usually then works its way into commercial application." A hypersonic airliner, for example, could conceivably reach anywhere in the world within four hours.